At small angles of attack, air flows smoothly around an airfoil, providing lifting force through the difference in pressure across the top and bottom of the airfoil. As the angle of attack increases, the lift produced by the airfoil increases as well but only to a point. Increasing the angle of attack also increases the adverse pressure gradient on the latter half of the top surface, visible here as an increasingly thick bright area. Over this part of the surface, the pressure is increasing from low to high–the opposite of the direction a fluid prefers to flow. Eventually, this pressure gradient grows strong enough that the flow separates from the airfoil, creating a recirculating bubble of air along much of the top surface. When this happens, the lift produced by the airfoil drops dramatically; this is known as stall.
Tag: airfoil
Circulation Around an Airfoil
As a followup to yesterday’s question about ways to explain lift on an airfoil, here’s a video that explains where the circulation around the airfoil comes from and why the velocity over the top of the wing is greater than the velocity around the bottom. Kelvin’s theorem says that the circulation within a material contour remains constant for all time for an inviscid fluid. Before the airplane moves, the circulation around the wing is zero because nothing is moving. As shown in the video, as soon as the plane moves forward, a starting vortex is shed off the airfoil. As the plane flies, our material contour must still contain the starting position and thus the starting vortex. However, in order to keep the overall circulation in the contour zero, the airfoil carries a vortex that rotates counter to the starting vortex. This is the mechanism that accelerates the air over the top of the wing and slows the air around the bottom. Now we can apply Bernoulli’s principle and say that the faster moving air over the top of the airfoil has a lower pressure than the slower moving air along the bottom, thus generating an upward force on the airfoil. (submitted by jessecaps)
Reader Question: How Airfoils Produce Lift
doughboy3-deactivated20120305 asks:
I’m a Undergrad Aeronautical Engineering student. I’m curious as to your opinion as to how airfoils produce lift. I know the usual theory told in this situation. However my aerodynamics professor says that there are many things going on during the flow around an airfoil. I’m hoping to get a better idea of the different mechanisms responsible for lift.
There’s a common misconception of Bernoulli’s principle that’s often used to explain how an airfoil creates lift (which I assume is the “usual theory” to which you refer), and while there are many correct (or, perhaps, more correct) ways of explaining lift on an airfoil, I think the only opinions involved are as to which explanation is best. After all, opinions don’t keep a plane in the air, physics does!
I tackled the air-travels-farther-over-the-top misconception and presented one of my preferred ways of looking at the situation in a previous post; in short, the airfoil’s shape causes a downward deflection of the flow, which, by Newton’s 3rd law, indicates that the air has exerted an upward force on the airfoil. There’s a similar useful video from Cambridge on the topic here.
Another explanation I have heard used concerns circulation and its ability to produce lift (see the Kutta-Joukowski theorem for the math). In this case, it’s almost easier to think about lift on a cylinder instead of lift on a more complicated shape like an airfoil. If you spin a cylinder, you’ll find that the circulation around that object results in a force perpendicular to the flow direction. This is called the Magnus effect and, in addition to explaining why soccer balls sometimes curve strangely when kicked, has been used to steer rotor ships. One of my undergrad aero professors used to do a demonstration where he’d wrap a string around a long cardboard cylinder and demonstrate how, by pulling the string, the cylinder’s spinning produced lift, making the cylinder fly up off the lectern and attack the unsuspecting students.
An airfoil doesn’t spin, but its shape produces the same type of circulation in the flow field. Without delving into the mathematics, it’s actually possible through conformal mapping and the Joukowski transform to show that the potential flow field around a spinning cylinder is identical to that around a simple airfoil shape! Although that mathematical technique is not all that useful in a world where we can calculate the inviscid flow around complicated airfoils exactly, it’s still pretty stunning that we can analytically solve potential flow around (and thus estimate lift for) a host of airfoil shapes on the back of an envelope.
In short, your aerodynamics professor is right in saying that there are many things going on during the flow around an airfoil. If you get a roomful of aerodynamicists together and ask them to explain how airfoils generate lift, you would be faced with a lively discussion with about as many competing explanations as there are participants. As you learn more in your classes, you’ll gain a better intuitive feel for how it works and you’ll learn more of the nuances, which will help you understand why there is no one simple-to-understand explanation that we use!**
** Lest I confuse someone into thinking that aerodynamicists don’t know how airfoils produce lift, let me add that the argument here is over how best to explain the production of lift, not over how the lift is produced. We have the equations to describe the flow and we can solve them. We know that lift is there and why. We simply like to argue over how to explain it to people without all the math.
Separation and Stall
This flow visualization of a pitching wind turbine blade demonstrates why lift and drag can change so drastically with angle of attack. When the angle the blade makes with the freestream is small, flow stays attached around the top and bottom surfaces of the blade. At large (positive or negative) angles of attack, the flow separates from the turbine blade, beginning at the trailing edge and moving forward as the angle of attack increases. The separated flow appears as a region of recirculation and turbulence. This is the same mechanism responsible for stall in aircraft. (Submitted by Bobby E)
Airfoil Boundary Layer
This video shows the turbulent boundary layer on a NACA 0010 airfoil at high angle of attack (15 degrees). Notice how substantial the variations are in the boundary layer over time. At one instant the boundary layer is thick and smoke-filled and in another we see freestream fluid (non-smoke) reaching nearly to the surface. This variability, known as intermittency, is characteristic of turbulent flows, and is part of what makes them difficult to model.
Airfoil-shaped Ice
I discovered this interesting bit of icing a couple years ago near the foot of a waterfall in Ithaca, NY. The predominant wind was always heading toward the falls (left to right in these pictures), while the falls were always throwing spray up into the wind. The result was that ice airfoils (center) formed in the wake of each tree branch throughout most of the gorge (top).
xkcd and Lift
xkcd identifies a very common misconception about how airfoils work! (via Vinnchan and jasonas14) #
How Wings Create Lift
One of the topics in fluid dynamics almost everyone has come across is the explanation of how airplanes produce lift. Using Bernoulli’s principle–which relates velocity and pressure–and a picture of an airfoil, your average science text will say that a bit of air going over the top of the airfoil has to travel farther than a bit of air going under the airfoil, and that, therefore, the air over the top travels faster than the air under the airfoil.
Unfortunately, this is misleading and, depending on the wording, outright wrong! The hidden assumption in this explanation is that air that goes over the top and air that goes under the bottom have to reach the trailing edge of the airfoil at the same time. But why would that be? (As one of my profs once said, “There is nothing in physics that says there is Conservation-Of-Who-You-Were-Sitting-Next-To-When-You-Started.”)
Take a look at the video above. It shows an airfoil in a wind tunnel using smoke visualization to show how the air moves. Around the 0:25 mark, the video slows to show a pulse of smoke traveling over the airfoil. What happens at the trailing edge? The smoke going over the top of the airfoil is well past the trailing edge by the time the smoke going under the airfoil reaches the trailing edge!
It’s true that air goes faster over the top of the airfoil than the bottom and that this causes a lower pressure on top of the airfoil (as Bernoulli tells us it should) and that this causes an upward force on the airfoil. But which causes which is something of a chicken-and-egg problem.
A more straightforward way, in my opinion, of explaining lift on an airplane is by thinking about Newton’s 3rd law: for every action, there is an equal and opposite reaction. Take a look at the air’s movement around the airfoil as the angle of attack is increased around 1:00 on the video. Just in front of the airfoil, the air is moving upward. Just after the airfoil, the air is pointed downward. A force from the airfoil has pushed the air down and changed its direction. By Newton’s 3rd law, this means that the air has pushed the airfoil up by the same amount. Voila! Lift!
